WASHINGTON- Pakistan is advancing toward a sea-based missile capability and expanding its interest in tactical nuclear warheads, a leading American newspaper reported Sunday, citing international concern over the move to expand its atomic arsenal.

The development of nuclear missiles that could be fired from a Navy ship or submarine would give Pakistan “second-strike” capability if a catastrophic nuclear exchange destroyed all land-based weapons, The Washington Post said, quoting Pakistani and Western analysts. In reporting what it called was an acceleration of Pakistan’s nuclear and missile programme, the newspaper voiced oft-repeated western concern about the “vulnerability of those weapons in a country home to more than two dozen extremist groups.”

“The assurances Pakistan has given the world about the safety of its nuclear programme will be severely tested with short-range and sea-based systems, but they are coming,” Michael Krepon, co-founder of the Stimson Center, a Washington-based global security think tank, was quoted as saying. “A cardinal principle of Pakistan’s nuclear programme has been: ‘Don’t worry; we separate warheads from launchers.’ Well, that is very hard to do at sea.”

The Islamabad-datelined Post dispatch said anti-government protests in Islamabad to push Prime Minister Nawaz Sharif’s government to the brink of collapse underscored Pakistan’s instability. The political crisis was unfolding as Pakistan and India continued lobbing artillery shells across their border, in a tit-for-tat escalation that illustrated the continued risk of another war, it was pointed out.

For more than a decade, the paper said, Pakistan has sent signals that it’s attempting to bolster its nuclear arsenal with “tactical” weapons — short-range missiles that carry a smaller warhead and are easier to transport.

Over the past two years, Pakistan has conducted at least eight tests of various land-based ballistic or cruise missiles that it says are capable of delivering nuclear warheads, according to the report. Last September, Sharif, citing “evolving security dynamics in South Asia,” said Pakistan is developing “a full spectrum deterrence capability to deter all forms of aggression.”

The next step of Pakistan’s strategy includes an effort to develop nuclear warheads suitable for deployment from the Indian Ocean, either from warships or from one of the country’s five diesel-powered Navy submarines, the Post said, citing analysts.

In a sign of that ambition, Pakistan in 2012 created the Naval Strategic Force command, which is similar to the air force and army commands that oversee nuclear weapons.

“We are on our way, and my own hunch is within a year or so, we should be developing our second-strike capability,” said Shireen Mazari, a nuclear expert and the former director of the Institute of Strategic Studies Islamabad, and now a PTI leader.

Pakistan’s nuclear push comes amid heightened tension with US intelligence and congressional officials over the security of the country’s nuclear weapons and materials, the paper said. The Washington Post reported in September 2013 that US intelligence had increased surveillance of Pakistan in part because of concerns that nuclear materials could fall into the hands of terrorists.

State Department spokeswoman Jen Psaki, asked if the United States was concerned about a sea-launched Pakistani weapon, said it was up to Pakistan to discuss its programmes and plans. But, she said, “We continue to urge all nuclear-capable states to exercise restraint regarding nuclear and missile capabilities. We continue to encourage efforts to promote confidence-building and stability and discourage actions that might destabilise the region.”

During a visit to Washington for consultations with the Obama administration in July, the Post quoted Tariq Fatemi, Prime Minister Sharif’s senior foreign policy adviser, as saying the government had “no intention of pursuing” sea-based nuclear weapons.

“It is unclear how much direct knowledge Sharif’s government has about the country’s nuclear weapons and missile-development programmes, which are controlled by the powerful military’s Strategic Planning Directorate,” the dispatch commented. But the prime minister is the chairman of the country’s National Command Authority, a group of civilian and military officials who would decide whether to launch a nuclear weapon.

Pakistani military officials declined to comment on the nuclear programme, it said. They note, however, that a January report by the Washington-based Nuclear Threat Initiative (NTI) named Pakistan the “most improved” in safeguarding nuclear materials.

Analysts, according to the paper, say much about Pakistan’s programme remains a mystery. Western experts, for example, are divided over whether Pakistan has the ability to shrink warheads enough for use with tactical or launched weapons.

“They may have done so, but I can’t imagine it’s very reliable,” Jeffrey Lewis, a nuclear and non-proliferation scholar at the Monterey Institute of International Studies, was quoted as saying. Still, Lewis and other analysts say Pakistan is without doubt embarking on an ambitious multi-year strategy to enhance its nuclear arsenal and delivery systems.

In 2011, nongovernment experts interviewed by the Post estimated that Pakistan had built more than 100 deployed nuclear weapons. Now Pakistan’s fourth plutonium production reactor is also nearing completion, and while most assessments of the country’s warhead inventory have not changed much in recent years, analysts say Pakistan continues to produce weapons material and develop delivery vehicles, positioning itself for another spurt of rapid growth at any time.

“They are going to make as much fissile material as they possibly can and keep making as many warheads as they possibly can,” Pervez Hoodbhoy, a leading Pakistani nuclear expert and physicist, was quoted as saying.

India, which experts estimate has 80 to 100 deployed nuclear weapons, the report said.

“But concerns within Pakistan about India’s growing nuclear ambitions are helping to fuel Pakistan’s own advancements.

Much of India’s ballistic technology appears aimed at boosting its defences against China, not Pakistan, according to the Post. But the Pakistani military has been shifting the focus of the country’s nuclear programme over the past decade because of fears that Indian forces could use the threat of terrorism to launch a sudden cross-border strike.

India has a sizable advantage in conventional weapons, and its army is more than twice the size of Pakistan’s. And in recent years, the Pakistan Army says, more than one-third of Pakistan’s 500,000 soldiers have been focused not on the eastern frontier, but on battling militants on the region bordering Afghanistan.

So instead of working to enhance the range of its missiles, Pakistan is developing shorter-range cruise missiles that fly lower to the ground and can evade ballistic missile defences, analysts say.

Pakistan has repeatedly tested its indigenously produced, nuclear-capable, Babur cruise missile, which has a range of 400 miles and can strike targets at land and sea, military officials said. In 2011 and last year, Pakistan also tested a new tactical, nuclear-capable, battlefield missile that has a range of just 37 miles.

“This is the miniaturisation of warheads,” Mansoor Ahmed, a strategic studies and nuclear expert at Quaid-i-Azam University in Islamabad, was quoted as saying.

Maria Sultan, chairwoman of the Islamabad-based South Asian Strategic Stability Institute, said the short-range missile is designed as a signal to India’s military.

“We are saying, ‘We have target acquisition for very small targets as well, so it’s really not a great idea to come attack us,’?” Sultan said. “Before, we only had big weapons, so there was a gap in our deterrence, which is why we have gone for tactical nuclear weapons and cruise missiles.” Still, even a limited use of nuclear weapons on the battlefield would likely trigger a major retaliatory strike from India, Manpreet Sethi, a senior fellow at the New Delhi-based Centre for Air Power Studies, was cited as saying.

“The use of tactical nuclear weapons is not going to change an [Indian] offensive in any substantial way,” Sethi said. “Slow down, yes, but not stop.”

Hans Kristensen, director of the Nuclear Information Project at the Federation of American Scientists, said the fact that Pakistani and Indian analysts even debate the outcome of a limited nuclear exchange is cause for alarm.

“India and Pakistan have so many avenues into a conflict that could spin out of control and such a history,” Kristensen was quoted as saying. “The development of these weapons systems lowers the point where you could potentially see nuclear weapons come into use.”